The present invention relates generally to high-intensity, metal halide arc discharge lamps having fused silica arc tubes filled with a mixture including sodium halides and at least one additional metal halide, and optionally mercury. More particularly, it relates to a borosilicate glaze present on the inner surface of the arc tube, the outer surface of the arc tube, or both the inner surface and the outer surface of the arc tube, for extending the useful life of the lamp by reducing the loss of the metallic portion of the fill and the corresponding undesirable buildup of free halogen in the arc tube which results from sodium ion diffusion through the fused silica arc tube or metal halide reaction with the fused silica arc tube.
Metal halide arc discharge lamps having a construction typical of this type of lamp are shown, for example, in U.S. Pat. Nos. 4,047,067 and 4,918,352 (electroded), and 5,032,762 (electrodeless). Metal halide lamps of this type generally contain a filling of light emitting metals including sodium and rare earth elements in the form of halides, commonly the iodide, and optionally mercury, in arc tubes composed of, for example, fused silica, alumina, and crystalline synthetic sapphire.
The lifetime of such lamps is frequently limited, however, by the loss of the metallic portion of the metal halide fill during lamp operation due to sodium ion diffusion and/or reaction of the metal halides with the fused silica arc tube and the corresponding buildup of free halogen in the arc tube. The term xe2x80x9cfree halogenxe2x80x9d as used herein refers to volatile forms of halogens or halogen containing molecules created in the normal operating lamp as a result of sodium ion diffusion through the arc tube wall or metal halide reaction with the fused silica arc tube. Such resulting free halogen products could include iodine gas (I2) or silicon tetra iodide (SiI4), respectively.
The mobility of the sodium ion is such that the arc tubes are somewhat permeable to it. During lamp operation, sodium will diffuse through the arc tube wall to the cooler region between the arc tube and the outer jacket of the lamp and deposit on the outer jacket and on arc tube support structure where present. The lost sodium is thus unavailable to the discharge and can no longer contribute its characteristic emission so that the light output gradually diminishes and the color shifts from white toward blue. The arc becomes constricted as sodium is lost and, in a horizontally operating lamp particularly, may bow against the arc tube wall causing it to soften, leading eventually to non-passive failure. Also, loss of sodium causes the operating voltage of the lamp to increase, often rising to the point where the arc can no longer be sustained, ending the life of the lamp.
An additional source of loss of the metallic portion of the fill and corresponding buildup of free halogen during lamp operation is the chemical reaction of metal halides in the fill with the silicon dioxide, SiO2, of the inner surface of the fused silica arc tube producing, for example, metal silicate crystals on the arc tube wall and free silicon tetra iodide. This results in a color shift in the lamp, arc tube wall darkening and/or cracking, plus lumen loss.
Thus, the industry has been searching for ways to prevent or minimize sodium loss by diffusion through the fused silica arc tubes of metal halide arc discharge lamps, as well as to reduce or prevent reactions of the ionizable, light-emitting metal halide species in the fill with the fused silica walls of the arc tubes. Attempts to solve these problems have included providing aluminum silicate and titanium silicate layers on the outside surface of the arc tube, as in U.S. Pat. Nos. 4,047,067 and 4,017,163, respectively. U.S. Reissue Pat. No. 30,165 discloses vitreous metal phosphates and arsenates as coatings for the inner surfaces of ceramic and silica arc tubes. U.S. Pat. No. 3,984,590 discloses aluminum phosphates and U.S. Pat. No. 5,032,762 discloses beryllium oxide as coatings for the inner surfaces of arc tubes.
Despite the coating advances of the prior art, the problems of loss of the light-emitting, metal halide portion of the fill by diffusion or reaction and the corresponding buildup of free halogen in the arc tube have not been heretofore satisfactorily solved.
Accordingly, the present invention provides a means for reducing loss of the metallic portion of the fill of an arc tube of a metal halide arc discharge lamp as a result of diffusion and/or reaction and hence provide a means for reducing the corresponding buildup of free halogen, thereby extending the useful life of the lamp.
This invention also provides a means to decrease UV emissions from the lamp by providing a glaze containing a UV absorbing species.
The present invention further provides a means to alter light or energy emission from the lamp by absorbing select wavelengths, i.e. UV or IR.
The present invention is directed to an improved arc tube and an improved metal halide discharge lamp including the improved arc tube having the aforesaid means.
The present invention is an improved arc tube of fused silica for an arc discharge lamp. Such an arc discharge lamp could be a metal halide arc discharge lamp, including a fill for the arc tube capable of initiating and sustaining an electric arc discharge, wherein at least one component of the fill reacts with the fused silica or diffuses through the arc tube walls. The fill will generally comprise a sodium halide, at least one additional metal halide, and an inert starting gas. The improved arc tube will generally comprise a tube of fused silica having an inner wall defining an arc chamber, the inner wall, the outer wall, or the outer and inner walls, of the tube having provided thereon a borosilicate glaze which is vitreous and light-transmissive. The borosilicate glaze is comprised of a borosilicate glass containing at least one metal oxide selected from aluminum, scandium, yttrium, and the rare earth elements. The borosilicate glaze may further contain additional rare earth elements or transition metals to alter the light or energy emission of the lamp by absorbing select wave lengths. For instance, titanium, cerium, cobalt, chromium, iron or neodymium may be added. Of course, combinations of the foregoing may also be added to obtain desired emissions. The borosilicate glaze has been found to effectively extend the useful life of metal halide arc discharge lamps by reducing loss of the metallic portion of the fill through diffusion and/or reaction, and thus reducing the corresponding buildup of free halogen. In a broader sense, the invention relates to a fused silica article having a glaze of such borosilicate on at least a portion of a surface thereof.
The present invention additionally provides a metal halide arc discharge lamp assembly, having an arc tube of fused silica for containing a plasma arc discharge, and having a borosilicate glaze provided on the inner surface, the outer surface, or both the inner surface and the outer surface of the arc tube, the borosilicate glaze being vitreous and light-transmissive, and being comprised of a borosilicate containing at least one metal selected from aluminum, scandium, yttrium, and the rare earth elements. The borosilicate glaze may further contain additional rare earth elements or transition metals to alter the light or energy emission of the lamp by absorbing select wave lengths. For instance, titanium, cerium, cobalt, chromium, iron or neodymium may be added. Of course, combinations of the foregoing may also be added to obtain desired emissions. As obvious to those skilled in the art, such a borosilicate glaze would improve the arc tube in both an electroded metal halide arc discharge lamp and a high intensity discharge electrodeless lamp which operates by radio or microwave frequency.
The present invention additionally provides the process of protecting a fused silica arc tube of a metal halide arc discharge lamp, the lamp containing a fill including sodium halide, at least one additional metal halide, and an inert starting gas disposed within the arc tube from loss of the metallic portion of the fill through diffusion and/or reaction, and a corresponding buildup of free halogen in the arc tube. The process comprises providing the inner surface, the outer surface, or both the inner surface and the outer surface of the arc tube with a borosilicate glaze which is vitreous and light-transmissive, and which is comprised of a borosilicate containing at least one metal selected from the group consisting essentially of aluminum, scandium, yttrium, and the rare earth elements. The borosilicate glaze may further contain additional rare earth elements or transition metals to alter the light or energy emission of the lamp by absorbing select wavelengths. For instance, titanium, cerium, cobalt, chromium, iron or neodymium may be added. Of course, combinations of the foregoing may also be added specific to desired emissions.